Unsaturated polyester resin compositions are finding widespread use in industry for manufacturing parts. One particular area, the automotive industry, is increasingly utilizing unsaturated polyester resins to manufacture body panels. The use of such resins reduces weight and eliminates corrosion problems associated with metal body panels. In such application, as with others, one criteria determinant as to whether or not a polyester resin composition is satisfactory is whether the surface smoothness of the finished part is adequate.
It is generally recognized that the surface quality of a molded article may be measured by optical enchancement technology to measure surface waviness. For the automotive industry, the surface quality must meet a stringent standard defined generally as a Class A surface. A known instrument, presently commercially available, for measuring surface waviness is a Diffracto (TPS-2) Test Plaque Station, available from Diffracto Ltd., Windsor, Ontario. To be qualified as Class A, the surface must have a Diffracto TPS-2 reading of less than 100, as generally recognized in the molding industry.
It is further recognized that the surface quality may be improved after molding the article by grinding, buffing, coating, or other means. However, it is clear that such added manufacturing steps add significantly to the cost of manufacture. It is preferable that an article satisfy the Class A surface requirements as molded. To achieve this, the composition must be capable of conforming to the surface of the mold throughout the curing process.
It is known that unsaturated polyester resins are subject to shrinkage during curing. The adverse effect on surface quality is also well documented. The shrinkage is known to cause fractures, warpage, sink, and other aspects of poor surface quality. Thus, much development work has been directed to reducing or eliminating the shrinkage during curing of unsaturated polyester resin compositions to improve the surface quality. Most of this work has been directed to the use of low-profile and low-shrink additives.
In formulating an unsaturated polyester resin composition, a second important criterium is the curing temperature. Low temperature curing resins are generally used in resin transfer molding (RTM). These resins generally cure at temperatures of room temperature up to approximately 71.degree. C.
For the RTM process, molds may be manufactured from products such as Epoxy, other polyesters, or gel-coated polyesters. These molding materials generally restrict molding temperatures to approximately 71.degree. C. or lower. The advantages of these molding materials are the low cost, less weight and short time of fabrication.
In contrast, higher temperature curing resins, relative to RTMs, require molds which are manufactured from steel or other metals. Well-known higher temperature tooling materials include aluminum, Kirksite alloy, and steel. The disadvantage of these molds is the cost associated with fabrication.
An unsaturated polyester resin composition is generally classified on the basis of its curing temperature. For purposes here, these categories include low temperature curing polymers which are capable of resin transfer molding in contrast to higher temperature curing formulations which cure above approximately 71.degree. C. Within each category, the shrinkage is also an important criterium for successful molding of Class A surfaces.
Low temperature curing unsaturated polyester resin compositions are known to be produced utilizing saturated dicarboxylic acids such as isophthalic acid or its derivatives or dicyclopentadiene (DCPD) as raw materials. They are generally believed unsatisfactory for molding Class A surfaces. It is further believed that the use of low-shrink additives or low-profile additives to reduce shrinkage is unsatisfactory with these resins. Although the morphology of a curing polyester is maintained with low-shrink additives, the reduction in shrinkage is insufficient for molding Class A surfaces especially at the low temperatures usually used in RTM.
Low-profile additives lead to a significant, easily recognized morphology. These additives cause a phase separation that leads to a microscopic void formation so that shrinkage is relieved internally. High reactivity unsaturated polyester resins, for example those with little or no saturated dicarboxylic acids, or dicyclopentadiene, are well known to be needed for low-profile systems. A recognized problem with low-profile resins is the need to be cured at well controlled and higher temperatures. The high temperature and high reactivity resins are needed so that the morphological changes (i.e., phase separation and void formation) are able to take place. These temperatures generally exceed 71.degree. C. These systems are well suited for sheet molding compounds, bulk molding compounds, and other processes where the molding is done in metal molds.
It is well-recognized in the thermoset industry that unsaturated polyesters can be mixed with monomers. Kingston (U.S. Pat. No. 4,499,142) discloses an isophthalic neopentyl glycol polyester which utilizes both methylmethacrylate and styrene as a cross-linking monomer. Kingston, however, discloses curing the composition of his invention at temperatures of 280.degree. F. (138.degree. C.). Kingston discloses that the use of methylmethacrylate and styrene substantially reduce smoke production at elevated temperatures when such particular cross-linking monomer mixtures are used in combination with dimethylmethylphosphonate. As documented in the tests included in Example I of the Experimental section such resin does not provide shrinkage control at RTM curing temperatures.
Obara et al. (U.S. Pat. No. 4,309,519) describe a process for producing a flyback transformer that will not delaminate or shrink when exposed to high voltage. The process includes treating the flyback transformer with a low-shrinkage resin composition.
The low-shrinkage resin composition includes a polyester produced by an addition-condensation reaction of a partially esterified dicyclopentadiene (DCPD) carboxylic acid. The DCPD carboxylic acid is obtained by reacting at 150.degree. C. or lower, a reactant group that includes DCPD, maleic acid, and water or hydroxylated DCPD and maleic anhydride. The reactant group may also include a hydroxylated DCPD and maleic anhydride, with one or more polyhydric alcohols.
The low-shrinkage resin composition is also described as including a monomer having one or more vinyl groups such as styrene and a low-shrinkage agent. The low-shrinkage agent is described functionally as an agent that makes the resulting resin composition having a low-shrinkage. Examples of such an agent include polystyrene, a polyvinyl acetate, a saturated polyester and the like. The curing of a coated article is disclosed as including curing at 25.degree. C. for 2 hours and 105.degree. C. for 7 hours.
Bayha (U.S. Pat. No. 4,447,577) discloses an emulsion of a polyester that is formed from dicyclopentadiene. The emulsion includes up to 75-80% water by weight. The polyester is described as being formed from dicyclopentadiene, maleic acid or maleic anhydride, and a compound which contains at least two alcoholic hydroxyl groups and which may be an alcohol, a glycol, or a polyol having at least three alcoholic hydroxyl groups. The patent also includes a step of dissolving this resin with 50% by weight of styrene. The resin is curable at room temperature. Linear shrinkage is indicated to be 5.8.times.10.sup.-3 inches/inch, or 0.58%, recognized as far outside parameters for producing a Class A surface.
Bayha (U.S. Pat. No. 4,551,489) also describes an emulsion that includes 50-80% by weight of water and 50-80% by weight of a polyester that is formed from dicyclopentadiene. The emulsion is also described as including a curing agent and a cross-linking agent for the polyester. A preferred cross-linking agent is described as styrene. The polyester is described as being made from maleic acid or maleic anhydride that is reacted with dicyclopentadiene to produce an intermediate moiety. The reaction is also described as preferably including a heat transfer agent such as a glycol. As recognized above, the shrinkage upon curing is too extensive to result in Class A surface.
Gardner (U.S. Pat. No. 4,626,570) describes a polyester resin that has features of being low-shrinking and low viscosity. The resin is made by contacting a material such as maleic acid or maleic anhydride with an organic polyol for a time and temperature sufficient to form a composition having a carboxylic acid terminated polyester. Further, a thermoplastic low-profile additive is added for shrinkage control and considered an essential element. As recognized in Table A of the patent, the resins of Gardner must be cured at temperatures of 138.degree. C. to 152.degree. C. to achieve proper curing.
Bansleben et al. (U.S. Pat. No. 5,082,878) disclose a shrinkage control agent for unsaturated polyester resin systems that are copolymers of vinyl versatate and vinyl acetate. The patent describes the unsaturated polyesters themselves as being prepared with anhydrides that include maleic anhydride and polyols that include glycol. The patent also describes the use of derivatives of dicyclopentadiene and glycerol. The patent further describes the use of styrene as a suitable cross-linking monomer.
The patent describes its shrink control agent as copolymers of fatty alkyl vinyl esters and short chain vinyl esters. By "fatty alkyl vinyl esters" the patent includes vinyl esters with a fatty alkyl chain of about 9 to 11 carbon atoms. See Col. 3, lines 26-42.
Ross et al. (U.S. Pat. No. 5,089,544) disclose a resin system having four components that reduce the possibility of shrinkage and delamination. The first component includes an unsaturated polyester. The unsaturated polyester is described as being made of materials such as propylene glycol, water, DCPD and maleic anhydride. The second component is a low-profile thermoplastic polymer. Methylmethacrylate polymer is included within this low-profile thermoplastic polymer group. The third component is an unsaturated monomer which copolymerizes with the unsaturated polyester. The styrene monomer is included within this group. The fourth component is a polyoxyethane substitute. Molding cycles are disclosed as requiring temperatures of 121.degree. C.-176.degree. C.
Accordingly, the need exists for an unsaturated polyester resin composition capable of being resin transfer molded. The resin composition should be curable at temperatures below 71.degree. C., yet provide a Class A surface as molded. As documented above, it is generally recognized that low-profile additives require curing temperatures above those suitable for resin transfer molding and thus the need exists for a low-shrinkage resin composition which does not require such additives.
The present invention addresses these as well as other problems associated with existing resin transfer molding compounds. The present invention also offers further advantages over the prior art and solves problems associated therewith.